The invention pertains to the field of F.sub.2.sup.+ color center lasers.
Intense optical pumping of the 1S.sigma..sub.g .fwdarw.2P.sigma..sub.u transition of an F.sub.2.sup.+ color center, as required for laser action, produces a slow orientational bleaching of the F.sub.2.sup.+ color centers that is eventually debilitating to the laser action. An article entitled "The E.sub.2.sup.+ Center in KCl Crystals Part II: Optical Properties and Analogy with the H.sub.2.sup.+ Molecule", by M. A. Aegerter and F. Luty, Phy. Stat. Sol. (b), Vol. 43, pp. 245-261, 1971 has shown that direct, single step pumping of the 1S.sigma..sub.g .fwdarw.2P.pi..sub.u transition, for crystal temperatures higher than a few tenths of degrees K., gives rise to a non-radiative decay to the 2P.sigma..sub.u state which produces a reorientation of the color centers. By choosing an appropriate light polarization, such pumping can be used to return all the color centers to the desired orientation. However,the ultraviolet source required for such pumping tends to release electrons from other defects in the crystal, thereby discharging the F.sub.2.sup. + color centers. As a result single step reorientational pumping is not a truly satisfactory method for repetitive reorientation in a color center laser.